Film thickness sensor having a porous blower

ABSTRACT

A sensor system for measuring the thickness of flat material that is moved relative to the sensor system has a first sensor device for measuring the thickness of the flat material and a device for generating an air cushion. The device is disposed in such a way that there is an air cushion between at least one side of the sensor device that faces the flat material, and the flat material, during operation. In the region of the air cushion, the first sensor device includes surface sections having porous material and/or material that is provided with micro-holes.

CROSS-REFERENCE TO RELATED APPLICATION

This is a national stage of PCT/EP2006/010230 filed Oct. 24, 2006 andpublished in German.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a sensor system for measuring the thickness offlat material, which is moved relative to the sensor system a blown filmline for manufacturing film which blown film line comprises such asensor system, and a method for operating a blown film line comprisingsuch a sensor system.

2. Description of the Prior Art

Sensor systems for measuring the thickness of flat material, which ismoved relative to the sensor system, have been disclosed in the priorart. They are preferably used for measuring the thickness of freshlyextruded film. The measured values serve for regulating the filmthickness. Therefore, in this context, the term “flat material” is meantto connote predominantly film-shaped or web-shaped material, which ismostly guided past the sensor system comprising a sensor device such asa sensor head.

The measuring methods used for measuring the film thickness includeoptical, radiometric, inductive, and capacitive measuring techniques.However, especially when producing sensitive or sticky films, sensorsystems that come into contact with the film cause damages on thelatter. EP 591 239 B1 therefore suggests distancing a sensor device fromthe film by means of an air cushion.

The sensor device, which is disclosed in EP 591 239 B1 and is distancedfrom the film in such a manner, is a capacitive sensor, the twoelectrodes of which are mounted on a sensor head on one side of thefilm. Both electrodes comprise active surfaces that are facing the film.An electric field, which penetrates the air cushion between the sensordevice and the film, the film itself, and the space that is locatedbehind the film and is likewise filled with air here, is also formedbetween these active surfaces of the two electrodes. Here, there isknown to be a dependence of the capacitance of the capacitor on thematerial-specific dielectric constant ∈_(τ) of the materials (air andfilm material) penetrated by the electric field. A change in thethickness of the film material thus changes the capacitance of thecapacitor. However, it has been seen that variations in the distancebetween the film and the active surfaces of the electrodes also changethe capacitance of the capacitor. This influence of the distance of thefilm—or any flat material—also exists in the other measuring principlesmentioned above such as the inductive measuring principle.

However, in blown film lines, in particular, this distance changesconstantly since the film flutters, which can also result in the sensordevice coming into contact with the film.

DE 195 11 939 A1 therefore suggests constantly measuring the distancebetween the sensor device and the film, regulating the position of thesensor device based on these measured values, and constantly adapting tothe fluttering movement of the film by moving the sensor device in theradial direction of the film bubble. However, it is additionallynecessary here to draw in the film using low-pressure nozzles in theboundary areas of the sensor device and thus to restrict the flutteringof the film relative to the sensor device.

Another approach for the same problem is the object of EP 801 290 B1. Itis likewise suggested here to regulate the distance between the sensorand the film permanently, the measurement signal for regulating saiddistance being acquired by measuring the stagnation pressure between thefilm and the sensor device. Since the movement of the sensor device ismore inertial than that of the fluttering film, it is further suggestedto reduce the errors resulting in thickness measurement from variationsin the distance, as follows: The actual distance between the sensor andthe film at the time of thickness measurement is measured. The(erroneous) measured value of thickness is corrected based on themeasured value of distance with the help of an error function.

Another possibility consists in measuring the film thickness only whenthe correct distance between the sensor and the film has just passedthrough. However, this results in irregular time intervals between themeasurements.

It is clear from this explanation that the measuring devices describedabove are complicated, expensive, and yet error-prone.

It is therefore the object of the present invention to redress thesedisadvantages.

SUMMARY OF THE INVENTION

This object is achieved by the characterizing features of the inventionas described herein. The present invention utilizes the fact that aircushions have proved to be substantially more stable in experimentsperformed on porous material or material that is provided withmicro-holes than on other materials.

The variations in the distance between the sensor and the film thus playa smaller role. The sensor can therefore be positioned more closelyagainst the film. It is possible to further reduce the fluttering of thefilm if that side of the sensor device that faces the film or any flatmaterial, in general, is pressed against the flat material properly.Usually, this material will then apply a counterforce, which counteractsthe stable air cushion of the invention. As a result, a state ofequilibrium can then be formed, which limits the fluttering of the film.Depending on the application, said counterforce can also often beapplied at least partly by objects or physical variables other than theflat material. In the case of a blown film line, the internal pressureof the film bubble plays a decisive role here.

In this application, it is advantageous to press into the film bubblewith that surface of the sensor device that faces the film bubble over alonger period of time—perhaps during the entire job. Here, the aircushion of a sensor device, which performs the measurement from outsidethe film bubble, is located in the radial direction of the film bubblewithin the nominal radius of the same. The same often applies to thatside of the sensor device that faces the film and even to parts of thesensor device itself.

The cross-section of the film bubble can get considerably deformed atthe measuring point. Usually, such sensors travel around the film bubblein the circumferential direction of the same in order to measure thethickness profile of the bubble along the circumference. The penetrationdepth of the air cushion into the nominal radius of the bubble canadvantageously range from 1 to 5 centimeters. It can also range from 5to 15 centimeters. The interesting feature in this development of theinvention is that the penetration depth does not come about as a resultof a position control process of the sensor device, in which the sensordevice attempts to follow the fluttering film and temporarily assumessuch a position. Rather, the pressure originating from the air cushion,for its part, influences the position and the fluttering behavior of thefilm.

In a particularly advantageous development of the invention, it istherefore possible to dispense with the entire effort involved in theposition measurement of the film, the constant rapid corrections in theposition of the sensor device, and other measures that have beensuggested in EP 801 290 B1 and DE 195 11 939 A1 in order to prevent theconsequences of the fluttering of the film.

Just when the sensor device assumes such a prominent position inrelation to the flat material and optionally exerts pressure on the flatmaterial, it is advantageous to detect when and whether defective pointsof the flat material approach the sensor device due to the mutualrelative movement. Otherwise the sensor device could create a hole atsuch a defective point or enlarge any such hole and reach into thematerial. The movement of the material relative to the sensor devicethen results in serious damage to the flat material and/or the sensordevice.

It is therefore advantageous, if appropriate, to detect such damages andto withdraw the sensor device from the material.

The application of the teaching of the invention to sensor devices,which comprise inductive or capacitive measuring means, appears to beparticularly advantageous since the influence of the variations in thedistance between the sensor and the film on the measurement results isparticularly significant in these measuring methods. The shape of theelectrodes in capacitive measuring devices, which have both electrodeson one side of the flat material, is shown in the three documents citedabove. Usually, the electrodes mutually encompass each other and theiractive surfaces are located on that surface of the sensor device thatfaces the flat material. The electrodes on this surface of the sensordevice are often two concentric circles or two ellipses or they have theshape of meandering segments, which are entangled in each other.

Sensor devices of the invention, which are used in blown film lines, canbe used at those locations of these lines in which it was hithertoimpossible to use these sensors. Until now, such sensors are disposed inthe conveying direction of the film between the calibration basket andthe flatness unit. The fluttering of the film is relatively low at thislocation; the film tube still has a round cross-section and is alreadycrystallized considerably.

One disadvantage of measuring the film thickness at this location is theconsiderable distance from the blown film die or cooling ring at whichdeviations arise in the film thickness and can also be regulated by theblowing of air or heating of the melt. It is possible using themeasuring system of the invention to measure the film thickness, forexample, between the die gap of the blown film die and the calibrationbasket, if such a die gap exists. The measuring system of the inventioncan therefore be advantageously combined with a device disclosed in thestill unpublished German patent application having the file number 102005 038 731.4. This document describes the manner in which thefluttering behavior of the just extruded film can be restricted directlyafter the extrusion of the film. The use of porous materials among otherthings is also suggested for this purpose. The “clamping” of the filmbetween two air cushions is also suggested. All measures suggested inthe document cited above and the features of the device for guiding thefilm or for restricting its fluttering behavior are regarded as part ofthe present document. Recourse to the disclosure of the afore-mentioneddocument within the scope of the present patent application ispermissible.

Another disadvantage of arranging the sensor device in the conventionalmanner is the high device-related expenditure resulting from separatelysuspending the sensor device (often at a height of several meters) andguiding the sensor device along the circumference of the film bubble. Itseems possible to combine the sensor device with the calibration basket.This is particularly advantageous in sensor devices of the invention dueto the afore-mentioned properties of the characteristic air cushion.

Several sensors can also be mounted along the circumference of thebubble, thereby sparing the sensor system the movement along thecircumference of the film bubble.

Sensor devices can also be mounted on the flatness unit. If the filmtube is actually flattened before it reaches the sensor, a sensor isusually only able to jointly measure the thickness of those twocircumferential sections of the film tube that are located on top ofeach other. However, this problem can also be addressed by means ofspecific calculation methods or a visual detection of the position ofthe boundary layer between the two layers.

Additional exemplary embodiments of the invention are defined in thedescription of the subject-matter and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the individual figures:

FIG. 1 is a sketch of a first blown film extrusion line

FIG. 2 is a sketch of a second blown film extrusion line

FIG. 2 is a sketch of a first blown film extrusion line

FIG. 3 shows a section taken along line A-A marked in FIG. 1 withadditional features of a sensor system extending around thecircumference of a film bubble

FIG. 4 is a plan view of that side of a capacitive sensor device thatfaces the flat material

DETAILED DESRIPTION OF THE PREFERRED EMBODIMENTS

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

FIG. 1 shows a first blown film extrusion line 4, in which a film bubble2 or a film tube is extruded by a blown film die 1 in the direction ofthe arrow z. The film bubble 2 is squeezed off by the squeezing device 7comprising nip rolls 5 and 6. The thickness measuring system 3 occupiesa typical position. Usually, such a thickness measuring system or sensorsystem 3 is a first sensor device or a first sensor head 14, which isguided around the circumference of the film bubble, as shown in FIG. 3.

FIG. 2 shows a second blown film extrusion line 4, which has the samefeatures as the first blown film extrusion line. However, in the secondblown film extrusion line, other partly necessary and partly optionaldevices are shown in order to depict their positions in an extrusionline.

The outer cooling ring 8, which blows cooling air against the film tube2, is the first component disposed in the conveying direction z. In moremodern extrusion lines, the outer cooling ring can regulate thetemperature of the cooling air, which is blown against circumferentialsections 21 n of the film tube 2 in order to even out thick places. Aregulation of the melt temperature in the blown film die is also triedout for this purpose.

The next additional feature of the extrusion line shown in FIG. 2 ascompared to the extrusion line shown in FIG. 1 is the calibration basket10, which contributes to determining the diameter of the film tube 2.

In FIG. 2, the sensor system 3 also comprises a second sensor device 24,whose function will be explained again later.

The flatness unit 11 is disposed before the squeezing device 7. Thefigure also shows the typical location of the frost region 9, in whichthe material existing initially as film melt is formed in an at leastpartly crystallized form or develops crystallites.

FIG. 3 is a sectional view of a sensor system 3 extending around thecircumference of a film bubble. The purpose of most of the componentsillustrated is the positioning and the movement of the first sensordevice 14 toward or in the vicinity of the film bubble 2. The arm 13,which is articulated to the sensor support 12 in such a way that saidarm can move in the radial direction “r” of the film bubble and servesfor directly holding the first sensor device 14. The sensor support 12,for its part, is displaceable together with the two aforementionedcomponents 13, 14 along the rail 22. Usually, the sensor support iscontinuously driven around the film bubble in the circumferentialdirection [(φ) direction]. For promoting a better understanding, thecircumferential sections of the film bubble 21 m and 21 o are shownagain in FIG. 3. They symbolize that modern thickness regulatingprocesses often break down the circumference of the film bubble into Nindividually adjustable circumferential sections or circumferentialsectors 21 n.

FIG. 3 does not show the manner in which the holding device formed ofthe arm 13 and the sensor support 12 positions that side 15 of thesensor head that faces the film or the air cushion 23 such that they arelocated within the nominal radius R of the film bubble 2. The distance Sof such a possible displacement or the (exaggerated) magnitude thereofis represented using the arrow marked by the letter S. The advantages ofsuch a displacement when using the sensor system 3 of the invention in ablown film line 4 have been discussed already. A second sensor device 24can also be included as a part of a preferred embodiment of a sensorsystem of the invention. The second sensor device 24 examines the filmfor damages and holes, as shown in FIG. 2, in the transport direction“z” of the film before the first sensor device 14. If the second sensordevice 24 detects such a hole, the first sensor device 14 is pulled awayfrom the film by a movement of the arm 13. The actuators for thismovement indicated by the arrow 25 can be mounted in the arm 13 or onthe sensor support 12. This movement of the arm helps prevent damages onthe film 2 and the first sensor device 14. Optical sensors or sensors,which react to electromagnetic radiation and which also often require acounterpart within the film bubble, are suitable for this purpose.

FIG. 4 shows a sketch of that side 15 of a first sensor device 14 of acapacitive sensor that is facing the film. The side 15 can be dividedinto the region outside the electrodes 16, the outer and the innerelectrodes 17 and 19, a dielectric 18 between these electrodes 17 and 19and often, but by no means always, an inner region 20.

Porous material or material that is provided with micro-holes can bepresent in each of these regions. Each of these regions can also beprovided with a material of such kind that air can be pressed through itin order to generate an air cushion. Such a material is good for thestability of the air cushion even if air is not pressed through thematerial directly at this location but only at neighboring locations.

At least one electrode, or at least the active surface thereof, couldalso be made of such a material. For this purpose, a metal—preferablysintered metal—could be selected in order to position the electric fieldin the most favorable manner possible.

It is advantageous in general and in connection with a pressing of thesensor into the film bubble, in particular, if the sintered material isfine-grained or if the material has fine holes. In this context, a grainsize or hole size of less than 100 μm is classified as fine. Grain sizesor hole sizes of less than 80 cm or even less than 50 μm are still moreadvantageous. In the case of such grain sizes or hole sizes, it ispossible to use air having relatively high pressure for generating theair cushion, which results in a stable air cushion. A relatively highpressure is ensured at 50 or 100 milliliter to one bar of overpressure.

The invention being thus described, it will be apparent that the samemay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be recognized by one skilled in the art areintended to be included within the scope of the following claims.

List of reference numerals  1 Blown film die  2 Film bubble/film tube  3Sensor system  4 Blown film line  5 Nip roll  6 Nip roll  7 Squeezingdevice  8 Outer cooling ring  9 Frost region 10 Calibration basket 11Squeezing device [sic: Flatness unit] 12 Sensor support 13 Arm 14(First) sensor device, sensor head 15 That side of the sensor devicethat faces the film 16 Region outside the electrodes 17 Outer electrode(active surface) 18 Dielectric between the electrodes 19 Inner electrode(active surface) 20 Inner region  21n Circumferential section 22 Rail 23Air cushion 24 Second sensor device 25 Arrow in the direction ofmovement of the sensor 14 away from the film 2 φ Circumferentialdirection of the film bubble r Radial direction of the film bubble zConveying or axial direction of the film bubble R Nominal radius of thefilm bubble S Distance by which the air cushion intervenes in thenominal radius of the bubble

1. A sensor system for measuring a thickness of a flat material that ismoved relative thereto, comprising: a first sensor device for measuringthe thickness of the flat material; a device for generating an aircushion between the flat material and at least one side of the firstsensor device that faces the flat material, the first sensor deviceincluding in a region of the air cushion, surface sections that includeat least one of a porous material and a material that is provided withmicro-holes; and a second sensor device for detecting damage or holes inthe flat material, the second sensor device being disposed relative tothe first sensor device and the flat material so as to detect the damageor holes in the flat material before the damage or holes reach theregion of the air cushion due to relative movement between the flatmaterial and the first sensor device.
 2. The sensor system according toclaim 1, further comprising a return motion device for changing adistance between the first sensor device and the flat material.
 3. Thesensor system according to claim 1, wherein the air cushion is locatedinside a nominal radius R of the film bubble.
 4. The sensor systemaccording to claim 1, wherein the first sensor device includes inductivemeasuring devices.
 5. The sensor system according to claim 1, whereinthe at least one of the porous material and the material that isprovided with micro-holes is disposed in at least one section of the atleast one surface of the first sensor device that faces the flatmaterial, selected from in a region between active surfaces of theelectrodes that face the flat material, around the active surfaces ofthe electrodes that face the flat material, and on the active surfacesof the electrodes that face the flat material.
 6. A blown film line thatincludes a sensor system according to claim 1, the sensor system beingconfigured to measure the film thickness in at least one location of theblown film line selected from between a blown film die and a frostregion of the film tube, at a calibration basket, between thecalibration basket and a flatness unit, at the flatness unit, at asqueezing device, and after the flatness unit.
 7. The blown film lineaccording to claim 6, further comprising a return motion device forchanging a distance between the first sensor device and the flatmaterial.
 8. A method of operating a blown film line for producing afilm with a sensor system that includes (a) a first sensor device formeasuring a thickness of the film, which is configured as a blown filmbubble during the measuring, (b) a device for generating an air cushionbetween the film and at least one side of the first sensor device thatfaces the film, the first sensor device including in a region of the aircushion, surface sections that include at least one of a porous materialand a material that is provided with micro-holes, and (c) a secondsensor device for detecting damage or holes in the film, the secondsensor device being disposed relative to the first sensor device and thefilm so as to detect the damage or holes in the film before the damageor holes reach the region of the air cushion due to relative movementbetween the film and the first sensor device, said method comprising:detecting with the second sensor device any damage or holes in the film;generating an air cushion between the film and at least one side of thefirst sensor device that faces the film; and measuring the thickness ofthe film from outside the film bubble with the first sensor device. 9.The method according to claim 8, wherein the side of the first sensordevice that faces the film is positioned by a holding device against thefilm bubble during a measuring period such that the surface of the firstsensor device that faces the film is positioned inside the nominalradius R of the film bubble.
 10. The method according to claim 8,wherein the air cushion (i) is generated by conveying air through theporous material or the material that is provided with micro-holes and(ii) is located inside a nominal radius R of the film bubble.